In the field of cardiac defibrillation, it is well known that the energy required to effectively defibrillate a human heart, varies with internal lead configuration and electrode placement, as well as with the responsiveness of a particular patient's heart. It is necessary to determine, with the highest degree of accuracy, the minimal energy level necessary to defibrillate a patient's heart using implanted defibrillation leads.(the defibrillation threshold).
One known method of determining the defibrillation threshold energy of an implantable system is to induce fibrillation of a patient's heart. Once fibrillation occurs, the heart is defibrillated through the implanted defibrillation leads. Initially, defibrillation is attempted at a relatively high energy level (high energy being used to ensure rapid defibrillation and hence minimize patient risk). If this energy level defibrillates the heart, the heart is placed in fibrillation again, and a defibrillation pulse of a lower energy level is applied to the heart. If the lower energy level defibrillates the heart, the process is repeated with even lower defibrillation pulse energy levels until the heart is not defibrillated. The defibrillation energy level for the permanently implanted device is then set, according to the physician's discretion, above that energy level which reliably defibrillates the heart.
A disadvantage of the aforementioned method is the need to repeatedly induce fibrillation in a patient's heart, and to repeatedly defibrillate the heart to determine the system thresholds.
Another method of determining defibrillation thresholds is set forth in U.S. Pat. No. 5,105,809, issued on Apr. 21, 1992. The method described in this patent begins by applying an initial electrical shock to the heart during a period of vulnerability, usually occurring contemporaneously with the T-wave of a conventional ECG. The energy level of the initial shock is sufficient high so as not to cause fibrillation. Assuming the initial shock fails to induce fibrillation, a second electrical shock is applied during a subsequent period of vulnerability, the second shock having a magnitude less than the initial shock. Subsequent shocks are then applied, each with a magnitude smaller than the preceding shock, until fibrillation is induced. When fibrillation finally occurs, the energy of the preceding shock (the last to not cause fibrillation), is deemed to be the energy level required to defibrillate via that particular lead configuration.
However, because the period of vulnerability differs from patient to patient (thus is not precisely known), and is not necessarily contemporaneous with the appearance of a T-wave, for best results the aforementioned procedure must be performed several times, each time corresponding to a different possible time interval of vulnerability. That is, the procedure is performed numerous times over distinct time intervals to insure that the shocks used to determine the defibrillation threshold were applied during a true period of vulnerability. This results in the patient being subjected to numerous shocks and several fibrillation episodes (though fewer than prior techniques) in an attempt to determine the defibrillation threshold.